The structure of the conventional rotary type crystal ball utilizing the spring of a music bell, or music box, as the power source is shown in FIG. 5. Because of its structural characteristics, such a design will have at least the following defects which are not easy to overcome.
1. When there is insufficient water pressure in the crystal ball, because the rubber plug is made of elastic material and is driven by the gears (not driven directly by the central axis), if the driving gear and the driven gear become slightly off-center or eccentric during assembly, the rotary disc in the drive chain will be unable to keep a normal position or will begin to oscillate. Thus it will not only cause the decorative article in the crystal ball to oscillate, but also will increase the resistance to the rotation of the driving shaft, thereby shortening the rotating life of the crystal ball.
2. The crystal ball is also affected by the heated expansion and cooled shrinkage of the water, which causes the state of insufficient water pressure as above described. If the environmental temperature around the crystal ball decreases, the water pressure in the crystal ball will decrease accordingly. This results in the bowing or hollowing inwardly of the rubber plug, whereby the above described oscillating state of the rotary disc will become more serious.
3. Because of the ready occurrence of the oscillating state and its visually unpleasant appearance, one considers matching the size of the decorative article on the rotary disc with the size of the rotary disc. In principle, it is usual to give the largest diameter to the rotary disc as possible. If a rotary disc having a smaller diameter is fitted with a decorative article having a diameter larger than the disc diameter, the article will easily ram into the fixed base at the edge of the rotary disc. On the other hand, the result of too large a rotary disc (usually the total size of the rotary disc and the edge of the fixed base is equal to the diameter of the rubber plug), will cause the rubber plug to suppress its flange to separate from fixed groove of the glass ball. This results from the close contact of the edge of the rotary disc to that of the rubber plug. When the clear water in the crystal ball expands from a higher environmental temperature, the rubber plug cannot effectively expand to protrude outwardly, whereby the clear water seeps out. After the clear water has seeped out, when the environmental temperature resumes to the original level or decreases, the water pressure will be insufficient, thus causing the defects of the oscillations of the rotary disc, as described in item 2.
4. As to the above described conventional crystal ball, in order to have a smooth and labor-saving rotation of the rotary disc, it can not be allowed to have too much deviation in the space between the driving shaft and the driven shaft when being assembled for operating. Hence the speed of assembling and its undesirable effects will be greatly affected by the result of such precise demands, Furthermore, the unit cost will be definitely increased.
5. The ratio of the manufacturing cost of the above structure to the selling price of the crystal ball itself is too high and does not correspond to the economics needed in the product's structure.